Selenium and Tellurium Concentrations of Carboniferous British Coals

Received: 7 November 2017 Revised: 18 April 2018 Accepted: 20 April 2018 DOI: 10.1002/gj.3238

RESEARCH ARTICLE

Selenium and tellurium concentrations of Carboniferous British coals

1 Department of Geology and Petroleum Geology, Meston Building, University of Standard mode and collision/reaction cell mass spectroscopy methods have been uti- Aberdeen, Aberdeen, UK lized in order to overcome spectral interferences and provide ultra‐low quantification 2 Trace Element Speciation Laboratory (TESLA), Department of Chemistry, University of selenium (Se) and tellurium (Te) in British Carboniferous coals for the first time. The of Aberdeen, Aberdeen, UK accurate detection of Se and Te in coals is becoming increasingly important, as coals 3 Departamento de Química, Universidade and pyrite have been identified as potentially significant trace element sources. The Federal de Santa Maria, Santa Maria, RS, Brazil mean Se concentration of British coals bear comparison to that of world coals, with 4 Centro de Ciências Químicas, Farmacêuticas e de Alimentos, Universidade Federal de anomalous Se content (concentrations above 4 mg/kg) across westerly exposures, Pelotas, Pelotas, RS, Brazil often coinciding with high sulphur (S) content and visible pyrite. New Te data for Brit- Correspondence Liam A. Bullock, Department of Geology and ish coals gives a mean concentration of 0.02 mg/kg, with anomalous Te in Ayrshire. Petroleum Geology, Meston Building, There is a positive correlation in the Te/Se ratio across the sample set. The close rela- University of Aberdeen, King's College, Aberdeen AB24 3UE, UK. tionship between Se and Te, as well as Se–Te with both early syngenetic and later Email: [email protected] cleat‐filling pyrite, confirms an important role for sulphides in Se and Te sequestration Funding information in British coals. The high Se‐Bowland Shale and/or Ordovician volcanics may have This work was supported by the Natural Environment Research Council (NERC), Grant/ provided the trace element source for British coals of similar or younger age. Regional Award Number: NE/L001764/1 intrusive activity (shallow tabular intrusions or more extensive plutons) and episodes

Handling Editor: I. D. Somerville of intense deformation can alter the thermal maturity of coals and may have driven the movement of trace element‐rich fluids through strata, locally enriching coals in Se and Te.

KEYWORDS

Carboniferous, coal, Coal Measures, Great Britain, pyrite, selenium, sulphur, tellurium

1 | INTRODUCTION Te in coals worldwide are particularly scarce. The global trend in environmental awareness and need for low carbon energy sources has led Previous research has detailed the relative abundance of trace ele- to an increasing interest in “strategic elements” such as Se and Te. ments in coal (Cravotta III, 2008a, 2008b; Dale, 2006; Davidson, These elements are essential for future green energy technologies 1996; Finkelman, Stanton, Cecil, & Minkin, 1981; Liu et al., 2006; due to their associated photovoltaic and photoconductive properties Liu, Wang, & Oakey, 2006; Pazand, 2015; Spears, 2015; Spears & (Ayres & Talens Peiró, 2013; Lusty & Gunn, 2014; STDA, 2010; Talens Zheng, 1999). However, due to analytical limitations, a complete Peiró, Méndez, & Ayres, 2011; Woodhouse et al., 2013). Coal has understanding of selenium (Se) and tellurium (Te) in British Carbonifer- been specifically identified as a promising source for future prospects ous coals is lacking. Because of its low abundance, available data for (Bullock, Parnell, Perez, & Feldmann, 2017; Bullock et al., 2018; Dai

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Geological Journal. 2019;54:1401–1412. wileyonlinelibrary.com/journal/gj 1401 1402 BULLOCK ET AL. and Finkelman, 2018; UKERC, 2012), and pyrite has been noted as a as Te) to coal strata of similar or younger age. Despite the extensive potential source of economic interest for Se (Keith, Smith, Jenkin, studies into trace elements in coal, there is still a limited understanding Holwell, & Dye, 2017). Both Se and Te occur in low abundances in of the processes of preferential accumulation of Se and Te in coal, the Earth's crust (Se = 0.05–0.09 mg/kg; Te = 0.002 mg/kg; Rudnick how they relate to coal properties, and their spatial and stratigraphic & Gao, 2003) but are known to be concentrated in specific geological distribution in British coalfields. Developments in analytical methods and environmental reservoirs and may be associated with mining for measuring Se and Te using cold plasma and collision/reaction cell activities and reserves (UKERC, 2012). These elements may also form (CRC) interface allow the detection of Se and Te down to ultra‐low in high concentrations in organic‐rich sediments, such as coal (in raw levels of detection in British coals. This study reports new data for form and as coal ash following combustion). An evaluation of concen- Carboniferous coals from across Great Britain (Figure 1) and, in partic- trations and controls on Se and Te occurrence in coal is important for ular, (a) establishes a database for Se and Te concentrations in a set of understanding resource potential and well as environmental threats coals of similar stratigraphic age; (b) assesses how variable or constant relating to their occurrence and processing. the concentrations are over a wide area; (c) assesses any variability in The enrichment of Se and Te in coals compared to crustal abun- the ratio of Te/Se; and (d) assesses any relationships between concen- dances reflects the affinity of these trace elements for both organic trations of Se and Te with percentage organic carbon, percentage matter and sulphur (Spears, 2015; Spears, Borrego, Cox, & Martinez‐ sulphur (S), and thermal maturity. Tarazona, 2007), and possibly for the addition of Se and Te during dia- Samples were analysed using both standard mode mass spectrom- genesis (Diehl, Goldhaber, & Hatch, 2004). Limited data also suggest etry methods and CRC microwave‐assisted wet digestion methods that trace element contents, including Se, increase with the thermal (Henn et al., 2018) for better detection of low concentrations. maturity of coal (Liu, Wang, & Oakey, 2006; Raask, 1985). High Se in British and Irish Carboniferous sediments has been previously iden- 2 | METHODS AND MATERIALS tified in organic‐rich black shales (Bowland Shale of mid‐Visean age; Parnell, Brolly, Spinks, & Bowden, 2016), and it was suggested that | Ordovician volcanics may have provided an anomalous source in the 2.1 Sampling and coal properties watershed. The high Se‐Bowland Shale and/or Ordovician volcanics Sample localities include exposures and collieries from the coalfields of may therefore provide a source of Se (and other trace elements such North West Scotland, across the Midland Valley of Scotland, Northern

FIGURE 1 Map of Great Britain, with Carboniferous exposed coalfields shown and sample localities (see Table 1) BULLOCK ET AL. 1403

England, the Midlands, Wales, and Kent (see Table 1 and Figure 1). In British coals have been collected from a range of sources (Armstroff, total, 44 seam sub‐samples were analysed for Se and Te content. Coal 2004; BCURA (The British Coal Utilisation Research Association), rank and thermal maturity indicator information has been compiled 2002; Burnett, 1987; DECC, 2013; Durucan, Ahsan, & Shi, 2009; based on vitrinite reflectance values. Vitrinite reflectance values for Parnell, 1992; Vincent & Rowley, 2004).

TABLE 1 Se and Te (mg/kg) concentrations, vitrinite reflectance (Ro), total organic content (TOC), and sulphur (S) composition of sampled British Coal Measures (measured on an as‐determined basis)

British Coalfield Location Formation Se (CRC) Te (CRC) Se (STM) Te (STM) Vit Refl (mean Ro) TOC (%) S (%) Fife‐Stirling Wemyss CG 6.0 0.110 ‐‐2.9 52.7 5.1 Fife‐Stirling Longannet CG 1.6 0.013 ‐‐0.6 50.2 0.3 Ayrshire Greenburn SCM 1.5 0.004 ‐‐0.6 71.8 1.7 Ayrshire Netherton SCM 0.7 0.020 ‐‐0.6 71.6 1.1 Cumberland Bing LCM 1.0 0.014 1.4 0.01 0.7 54.0 0.5 Cumberland Flimby LCM 0.4 0.004 ‐‐0.8 79.7 0.7 Yorkshire Kellingley UCM 1.0 0.014 1.8 0.02 0.7 60.5 2.2 Derbyshire Creswell MCM 0.3 0.012 ‐‐0.9 60.2 1.1 North Wales Point of Ayr LCM 1.7 0.020 ‐‐0.8 57.0 0.8 Nottinghamshire Cotham LCM 5.3 0.070 ‐‐0.5 64.8 0.9 Nottinghamshire Gedling LCM 0.3 0.009 ‐‐0.5 67.2 0.7 Leicestershire Asfordby LCM 3.6 0.030 ‐‐0.5 53.9 0.8 Leicestershire Minorca LCM 1.1 0.018 0.9 0.01 0.5 56.1 0.7 Staffordshire Lea Hall MCM 0.7 0.004 ‐‐0.6 63.4 0.8 Warwickshire Baddesley LCM 1.5 0.016 ‐‐0.5 47.1 0.8 Warwickshire Daw Mill WG 1.8 0.021 ‐‐0.6 49.0 0.9 South Wales Nant Helen LCM 2.9 0.047 ‐‐2.8 27.6 12.3 South Wales Cwmbargoed MCM 0.7 0.004 ‐‐1.1 67.7 1.0 South Wales Taff Merthyr SWUCM 2.2 0.026 ‐‐1.9 64.0 0.6 South Wales Cwm SWUCM 1.2 0.012 ‐‐1.4 72.5 0.5 Kent Tilmanstone WG 3.3 0.066 ‐‐1.5 77.9 0.4 Argyllshire Inninmore Bay SCM ‐‐1.8 0.04 1.4 66.4 0.6 Fife‐Stirling Manor Powis CG ‐‐1.3 <0.01 0.6 61.0 0.5 Fife‐Stirling Fishcross CG ‐‐1.4 <0.01 0.6 ‐‐ Argyllshire High Tiefergus CG ‐‐5.7 0.01 1.0 59.8 0.6 Ayrshire Greenburn SCM ‐‐5.3 0.01 0.6 30.2 27.5 Ayrshire Greenburn SCM ‐‐2.2 0.04 0.6 ‐‐ Cumberland Maryport LCM ‐‐3.3 0.09 0.8 ‐ 1.0 Cumberland Venture LCM ‐‐1.5 0.06 0.7 ‐ 1.9 Cumberland Haig UCM ‐‐1.4 0.05 0.8 ‐ 1.1 Cumberland Parton Bay UCM ‐‐3.4 0.05 0.7 ‐ 0.1 Cumberland Lowca Point UCM ‐‐2.6 0.07 0.8 ‐ 5.1 Northumberland Lynemouth MCM ‐‐1.0 0.02 0.7 ‐ 1.8 Northumberland Potland Burn MCM ‐‐1.0 0.01 0.7 62.1 0.4 Northumberland Whitley Bay MCM ‐‐0.5 0.01 0.6 62.4 0.4 Northumberland Shotton MCM ‐‐5.9 0.06 0.7 57.5 5.3 Yorkshire Fountain's Fell YG ‐‐0.6 0.01 0.7 71.0 0.4 Yorkshire Todmorden Moor LCM ‐‐0.6 0.01 0.7 50.0 0.3 North Wales Wrexham WG ‐‐5.6 0.01 0.7 67.6 4.2 Derbyshire Buxton LCM ‐‐1.1 <0.01 1.2 58.7 1.0 Staffordshire Hanley UCM ‐‐0.6 0.01 0.8 31.4 0.2 Staffordshire Apedale UCM ‐‐5.6 <0.01 0.8 65.9 3.0 South Wales Pembroke SWUCM ‐‐1.0 0.01 2.5 69.2 4.2 Somerset Midsomer Norton MCM ‐‐0.4 0.02 0.9 ‐ 0.2

Note. Hyphen (‐) indicates measurement not taken by method for sample. Method: CRC = collision/reaction cell ICP‐MS method; STM = standard mode ICP‐MS method. Formation: CG = Clackmannan Group; SCM = Scottish Coal Measures; LCM = Lower Coal Measures; YG = Yoredale Group; UCM = Upper Coal Measures; SWUCM = South Wales Upper Coal Measures; MCM = Middle Coal Measures; WG = Warwickshire Group; Se = selenium; Te = tellurium. 1404 BULLOCK ET AL.

2.2 | Se–Te determination or H2O2 were not required. The accuracy of MAWD method was evaluated using two CRMs: NIST 1635 (subbituminous coal) and SARM 20 The Se and Te whole‐rock content of coals was determined by two (coal). No statistical difference (t test, 95% of confidence level) was techniques: First, selected whole rock samples were analysed as part observed between results obtained for CRMs and the certified values. of a suite of 51 elements using standard mode aqua regia inductively A quadrupole‐based ICP‐MS (NexION 300X®, Perkin Elmer, coupled plasma‐mass spectrometry (ICP‐MS) techniques at ALS Canada) equipped with a concentric nebuliser, a cyclonic spray cham- Minerals (Loughrea, Ireland). Samples were milled and homogenized, ber, and a quartz torch with a quartz injector tube was used. In order and 0.25 g were digested with aqua regia in a graphite heating block. to avoid any damage to the nebulization system of the ICP‐MS instru- The residue was diluted with deionized water, mixed and analysed ment due to the HF content in digests, a dilution factor of 10 was used using a Varian 725 instrument (ICP‐MS method code ME‐MS41). The for sample analysis. The ICP‐MS instrument was equipped with a cell standard mode lower limit of detection for Se is 0.2 mg/kg and for Te that can be used both as a collision cell and as a dynamic reaction cell. is 0.01 mg/kg. Measurement of four Geological Certified Reference This collision/reaction cell (CRC‐ICP‐MS) method allows for lower Material (CRM) standards were within the anticipated target range levels of detection of Se and Te by minimizing the occurrence of spec- (upper and lower bound) for each metal and standard. The CRMs used tral and nonspectral interferences. With standard mode ICP‐MS for calibration were MRGeo08 (mid‐range multi‐element CRM), methods, spectral interferences occur at the most abundant isotopes GBM908‐10 (base metal CRM), OREAS‐122, and OGGeo08 (ore grade of Se. Here, hydrogen gas was utilized in order to effectively minimize multi‐element CRMs). Duplicate analyses of two samples produced the spectral interferences of the argon‐based polyatomic ions at m/z reported values within the acceptable range for laboratory duplicates, 77, 78, and 80. For Te, m/z 128 was monitored for spectral interfer- with a mean relative percent difference of 4%. Duplicate analyses ences. In order to limit polyatomic interferences with argon‐, matrix‐, included samples processed through the entire analytical procedure. and solvent‐based ions, Se determination must be carried out by mon- Second, selected samples were digested by microwave assisted itoring 78Se isotope. Under optimized conditions, the instrumental and wet digestion (MAWD) and Se and Te determined by ICP‐MS. Sam- method limit of detection was 0.01 μg/L, suitable for Se and Te deter- ples were ground using a cryogenic mill, with 2 min of pre‐cooling time mination at very low concentration in coal. The overall effectiveness followed by 3 min of grinding. Cryogenic milling was used to minimize and importance of the method for ultra‐low Se determination is the risks of contamination as it is performed in closed environment further detailed in Henn et al. (2018). (avoiding contamination by the atmosphere of laboratory) and provides a very fine particle size in a relatively short time, important for | the anticipated low levels of Se and Te in the coal sample set. Samples 2.3 Total organic carbon and sulphur content were oven dried at 105 °C for 2 hr before use. The microwave‐ Total organic carbon (TOC) and sulphur (S) contents were measured induced combustion method for coal analysis is comprehensively using a LECO CS225 elemental analyser, after decarbonatization with detailed by Flores et al. (2008). hydrochloric acid, to a precision of ±0.05%. Analyses were run concur- Coal was digested by MAWD using a microwave oven (Multiwave rently with CRMs 501‐024 (Leco Instruments, 3.23 ± 0.03% C, 3000®, Microwave Sample Preparation System, Anton Paar, Graz, 0.047 ± 0.003% S, instrument uncertainty ±0.05% C, ± 0.002% S) Austria, software version v1.27‐Synt) equipped with up to 16 PTFE and BCS‐CRM 362 (Bureau of Analysed Samples Ltd., 1.48% S). The vessels with 100 ml of internal volume and maximum operational tem- instrument was calibrated using these CRMs. The repeatability of perature and pressure of 220 °C and 40 bar, respectively. Experiments sample results was consistently within 1%, based on the three runs were carried out using 250 mg of sample (diluted to 25 ml). These of CRM certified versus obtained values, and uncertainty was deter- operating conditions were used to select suitable dilutions prior to mined based on the results of three blanks. Blank sample measure- Se and Te determination to avoid C interferences. ments provided the methodological interference response from three Hydrofluoric acid (40%) was used without previous purification analyte‐free samples. The resulting trace S and TOC values obtained for digestion by MAWD. Water was purified using a Milli‐Q system for the blank samples were then algebraically subtracted from the and used to prepare all standard solutions and reagents. Concentrated standard and sample responses to avoid analytical interferences.

HNO3 (65%) was distillated in a subboiling system for MAWD and preparation of reference solutions. Reference solutions of Se and Te (0.01 to 10 μg/L) were prepared by sequential dilution of a stock solu- 3 | STRATIGRAPHY AND SEDIMENTOLOGY tion (1,000 mg/L) in HNO3. Argon (99.998%) was used for plasma generation, nebulization and as auxiliary gas and hydrogen (99.999%) was The coal samples from British coalfields are all of Carboniferous age, used in the CRC. predominantly represented by Coal Measures. The Coal Measures In order to evaluate the MAWD method, select samples were Supergroup of the UK extends from the Midland Valley of Scotland, digested by MAWD using three digestion solutions: (a) 4 ml HNO3 +1ml across the north of England, to the Midlands, South Wales, and Kent.

HCl + 1 ml HF, (b) 4 ml HNO3 + 1 ml H2O2 + 1 ml HF, and (c) 7 ml The widespread development of coal‐bearing strata formed early in

HNO3 + 1 ml HF. No statistical difference (ANOVA) was observed the European Westphalian equatorial climatic (delta‐top) conditions between the results obtained for Se and Te among all evaluated (Fielding, 1982; Spears, 2015; Waters, Browne, Dean, & Powell, methods. These results indicate that the digestion solution (HNO3 + HF) 2007). The depositional environment was characterized by low‐lying was suitable for Se and Te recovery for coal and other reagents, and HCl paralic delta plains with abundant vegetation and major river BULLOCK ET AL. 1405 distributaries (Spears, 2015). The main coal‐bearing intervals comprise anthracitic. The arithmetic mean vitrinite reflectance value is 1.1, with the Lower and Middle Coal Measures, formed during the Westphalian typically higher thermal maturity values in South Wales (vitrinite A and Westphalian B to Lower C in a fluvial coastal plain environment reflectance values of 1.1–2.8) and Fife‐Stirling (2.9; Table 1). Samples (Gayer, Peŝek, Sykorova, & Valterova, 1996). This was dominated from Ayrshire, Northumberland, Cumberland, North Wales, and by overbank mudrocks, representing deposition in extensive Staffordshire contain visible pyrite, including disseminated and con- interdistributary lakes (Gayer et al., 1996). The start and end of the Car- cretionary banded (bedding‐parallel) pyrite, and discordant, typically boniferous (into the Stephanian) was marked by a climate that was (at vertical, cross‐cutting pyrite (i.e., cleat‐hosted pyrite concentrated least seasonally) arid (Waters et al., 2007). An increase in sand content within small fractures). Samples from Ayrshire include 1‐cm thick and a climatic shift to drier conditions led to a decrease in the number bands with a high pyrite content. Samples contain a mean moisture of coal seams during the Westphalian C, Westphalian D and Stephanian content of 4.8%, fixed carbon content of 60%, vitrinite of 78% maceral (Besly, Burley, & Turner, 1993; Kombrink, 2008). The Coal Measures content, and volatile matter content of 30% (BCURA, 2002). were strongly affected by post‐depositional Variscan fold and thrust deformation, and such deformation resulted in a set of superimposed tectonic fractures on the coals, partially obliterating a cleat fracture sys- 4.2 | Se–Te content of British coals tem (Frodsham, Gayer, James, & Pryce, 1993; Gayer et al., 1996; Gayer & Nemcok, 1994; Owen, 1974; Woodland & Evans, 1964). Although all Results are shown in Table 1 and Figures 3–9. The world mean Se con- the sampled coals are Carboniferous in age, there are several tent of coals has been previously estimated in several studies, gener- subformations and some age variations across the sample suite. Sam- ally giving a range of 1–3 mg/kg (Nalbandian, 2012; PECH, 1980; ples span the range (from youngest to oldest): Upper Coal Measures Valkovic, 1983; Yudovich & Ketris, 2006). Here, Se values in excess and South Wales Upper Coal Measures (Westphalian C in age), Middle of 4 mg/kg are considered anomalous. This is based on concentrations Coal Measures and Warwickshire Group (Westphalian B), Lower Coal known to lead to environmental implications (e.g., 3.6 mg/kg mean Se Measures and Scottish Coal Measures (Westphalian A), one Yoredale in British Columbia coals; Vesper, Roy, & Rhoads, 2008). Group sample (Namurian), and Clackmannan Group (Visean; Figure 2). The arithmetic mean British coal Se content for samples analysed in this study is 2.1 mg/kg, in line with the world mean content and slightly higher than the British mean of Spears and Zheng (1999). This value is slightly higher than the world mean Se content for bituminous 4 | RESULTS coals of 1.6 mg/kg (Yudovich & Ketris, 2006). Overall, samples with detectable Te show an increase with increasing Se, with a strong pos- | 4.1 Coal properties itive correlation (linear trendline R2 value = 0.85, calculated using The majority of British coal samples are bituminous in coal rank, Microsoft Excel; Figure 3). The standard mode method shows a mean although some samples from the South Wales Coalfield are Se content of 2.5 mg/kg, similar to that of the CRC method of 2.3 mg/ kg. Ayrshire (3.7 mg/kg) and North Wales (3.7 mg/kg) show the highest mean regional Se content from the sample set (Figure 4). Anomalous Se is recognized in individual coal sample localities from Fife‐Stirling (6 mg/kg), Northumberland (5.9 mg/kg), Nottinghamshire (5.3 mg/kg), Argyllshire (5.7 mg/kg), Ayrshire (5.3 mg/kg), North Wales (5.6 mg/kg), and Staffordshire (5.6 mg/kg). Figure 5 shows that high Se tends to occur in westerly coal exposures (e.g., Western Scotland, North and South Wales, Cumberland, and Staffordshire) as well as in Northumberland and Fife‐Stirling.

FIGURE 3 Plot of Se versus Te (mg/kg) for sampled British coals. FIGURE 2 Simplified Carboniferous stratigraphy for sampled British CRC ICP‐MS = collision/reaction cell ICP‐MS method; STM ICP‐ coal‐bearing strata (adapted from Waters et al., 2007) MS = standard mode ICP‐MS method 1406 BULLOCK ET AL.

Argyllshire typically show higher Te content (mean 0.03 mg/kg), and coal samples from Cumberland (max. 0.09 mg/kg, mean 0.05 mg/kg), Nottinghamshire (max. 0.07 mg/kg, mean 0.04 mg/kg), Northumberland (max. 0.06 mg/kg), and Kent (max. 0.07 mg/kg) also show higher values (Figures 4 and 6, Table 1). The mean Te content by standard mode method also compares well to the CRC method (0.02 and 0.03 mg/kg, respectively). Whilst Te values measured by both methods compare well, the three coals that were measured by both methods show some analytical variation with Se calculations: the Minorca (Leicestershire) sample shows a good agreement for Se of 1.1 (CRC) and 0.9 mg/kg (STM), whereas Bing (Cumberland) and Kellingley (Yorkshire) show slightly overestimated Se by standard mode (1.4 and 1.8 mg/kg, respectively) compared to CRC (1.0 mg/kg for both samples). Clackmannan Group and Visean age samples contain the highest mean Se and high Te (3.6 and 0.03 mg/kg, respectively), whereas the Westphalian samples show a similar range of mean Se (1.9–2.2 mg/kg) and Te (0.01–0.02 mg/kg) contents (Figure 7). Middle Coal Measures samples show the highest average Te (0.04 mg/kg). The Yoredale sample contains the lowest overall Se (0.6 mg/kg) and Te (0.01 mg/kg) for the sampled strata (Figure 7). There is a slight positive correlation FIGURE 4 Mean (a) Se and (b) Te content of sampled British between Se and S (Figure 8a), but no strong relationship between Se Coalfield regions and TOC (Figure 8b). There is no strong relationship between Te and S, nor Te and TOC (Figure 8c,d). There is very high S in Ayrshire coals The mean Te content of British coals is 0.02 mg/kg. The highest (27.5%), attributed to high pyrite content (completely pyritic seams in recognized Te content of any British coal sampled is from Fife‐Stirling coal). Northumberland, Cumberland, South Wales, Ayrshire, Fife‐ (0.11 mg/kg), an order of magnitude higher than the estimated world Stirling, Yorkshire, Staffordshire, North Wales, and Pembrokeshire mean Te value (Table 1). The Midland Valley of Scotland and contain anomalous S content and visible pyrite. Here, anomalous S

FIGURE 5 Se concentrations delimited by spot size (in mg/kg) of sampled British coals [Colour figure can be viewed at wileyonlinelibrary.com] BULLOCK ET AL. 1407

FIGURE 6 Te concentrations delimited by spot size (in mg/kg) of sampled British coals [Colour figure can be viewed at wileyonlinelibrary.com] concentrations are values considered above 1.3%, the cut‐off value for S Previously, Te was seldom recognized in coals worldwide due to content of coals acceptable for local power generation in its low abundance. The meanTe content of world coals has been taken Northumberland (Turner & Richardson, 2004). Figure 9 shows that some at 1/100 of world mean Se content, giving a mean world Te content of higher Se and Te content coals coincide with higher vitrinite reflectance 0.015 mg/kg (Davidson & Lakin, 1973). As a result (and with no known values (e.g., Kent, South Wales, Fife‐Stirling, and Northumberland). previous analysis of Te in British coals for comparison), any Te above this world mean value is considered anomalous, though concentrations are too low to be considered significant in any economic or 5 | DISCUSSION environmental sense. Limited previous data for Te in coals from China, Japan, Australia, and UK are all in the range 0.02–0.08 ppm (Hashimoto et al., 1989; Spears & Tewalt, 2009; Woo, Watanabe, 5.1 | Comparison to world coals Hashimoto, & Lee, 1987), similar to the values determined in British Results show that British coals contain typical Se concentrations coals. comparative to world Se coal content. Previous databases for Australia, the United States, UK, and Iran record mean Se values of 5.2 | Source and enrichment of Se and Te in British 1.02, 1.7, 1.8, and 1.2 mg/kg, respectively (Coleman, Bragg, & coals Finkelman, 1993; Pazand, 2015; Riley et al., 2007; Spears & Zheng, 1999). Elevated Se levels in coal have led to a suggestion that coal High Se in the Bowland Shale and underlying Ordovician volcanics might be a global resource for the element, of up to 500,000 tonnes, (Parnell et al., 2016) may have provided the initial source for Se and based on a content of 0.5 mg/kg (Speirs, Houari, Contestabile, Gross, Te for British coals of similar or younger age. This is particularly true & Gross, 2013). Two regions where coal waste is regarded as an envi- of upper Visean‐Namurian Clackmannan Group (Waters et al., 2007, ronmental problem, British Columbia (Wellen, Shatilla, & Carey, 2015) 2011) samples, which were deposited at a similar time to the Bowland and West Virginia (Lindberg et al., 2011), have mean Se contents of Shale. The limited overall Se and Te range of Westphalian A–C sam- about 2.6 and 3.6 mg/kg, respectively (Kennedy, Day, Mackie, & ples suggests there was little variation during the Westphalian period, Pesonen, 2015; Vesper et al., 2008). All of these mean values are which suggests that local variations in thermal maturity and minerali- lower than the 4 mg/kg level considered anomalous, above which 6 zation may play a role in the regional and stratigraphic trace element of the 15 British regions measured. The regions of high Se content patterns of enrichment throughout Great Britain. There are some cor- in British coals do not match to regions of anomalously high stream relations of Te and Se with high vitrinite reflectance (Figure 8), which water Se content (Salminen, 2005). may indicate that more thermally mature coals contain higher trace 1408 BULLOCK ET AL.

Dissolved metals in fluids can enhance Se and Te content of coals and have been suggested for Carboniferous coals in Ayrshire (Bullock et al., 2018) and Jurassic coals in Brora (Bullock et al., 2017). The close proximity of the South Wales, Cumberland, and Northumberland coalfields to granitic intrusions (e.g., Shap and Skiddaw granites of Cumberland, Dartmoor granite in southwest England, Weardale granite in northeast England), associated deformation which produced a series of faults through the coal strata (forming cleat systems), or later Variscan activity (in the case of South Wales), may have provided a source and/or flow pathway for hot metal‐rich fluids and subsequent increased maturity and trace element enrichment to these coals (see also Gayer et al., 1996; Turner & Richardson, 2004). The Clackmannan Group coals in Fife‐Stirling shows higher mean Se and Te, which may be attributed to the 18‐m thick Midland Valley Sill complex causing local heating of strata (Vincent, Rowley, & Monaghan, 2010), which may be responsible for the higher thermal maturity, and may have provided a potential source and/or fluid transport mechanism for chalcophile elements through the strata. Enrichment of Se (and possibly) Te regions such as Northumberland and the northern Pennines may be additionally influenced by seawater distribution, with periodic inundation by the sea into low‐lying deltaic planes (Cann and Banks, 2001; McKay and Longstaffe, 2003; Turner and Richardson, 2004; Bouch et al., 2006; Spears, 2015; 2017). There is also a positive correlation in the Te/Se ratio (Figure 2), suggesting a close relationship between the two elements, both substituting for S in pyrite. Selenium can substitute readily for S in many sulphide minerals and can also be precipitated by sulphate‐reducing microbes (Hockin & Gadd, 2003; Parnell, Bellis, Feldmann, & Bata, 2015). In coals, Te also has an indirect association with organic matter, FIGURE 7 Mean (a) Se and (b) Te content of sampled coal‐bearing and microbial activity can concentrate both Se and Te. High Se and strata Te coals recognized in this study typically contain visible pyrite (e.g., samples from Ayrshire, Northumberland, South Wales, and element content, previously attributed to uptake from groundwaters Staffordshire). The role of pyrite is therefore pivotal in sequestration during burial (Raask, 1985). of Se and Te in coals. However, the cross‐plot of Se versus S indicates Causes of thermal maturity in coal, such as igneous intrusions only a slight a positive correlation, with several outliers (Figure 8a). This heating the strata, may also be accompanied by trace element‐carrying may therefore suggest that pyritic S is important in Se and Te seques- hydrothermal fluids percolating through coals, precipitating as pyrite. tration, but sulphate and organic sulphur content in coals do not play

FIGURE 8 Cross‐plots showing (a) Se versus S, (b) Se versus TOC, (c) Te versus S, and (d) Te versus TOC. Se = selenium; Te = tellurium; TOC = total organic carbon BULLOCK ET AL. 1409

FIGURE 9 Vitrinite reflectance (mean Ro)of British coals (see text for refs.), with corresponding areas of high Se and Te content. Se = selenium; Te = tellurium

a pivotal role in Se and Te sequestration, resulting in the data point 6 | CONCLUSIONS outliers and a lack of correlation for total S versus Se and Te. Though Se and Te have an affinity to organic matter (binding to the surface), A combination of standard mode and collision/reaction cell ICP‐MS the lower trace element content in nonpyritic coal compared to analytical methods have led to the establishment of Se and Te concen- pyritic coal suggests that organic matter may elevate mean concentra- trations of Carboniferous British Coal Measures to ultra‐low levels of tions above world average crustal compositions but is unlikely to play detection. a significant role in enrichment compared to other coals and organic‐ Results show that rich sediments. 1. British coals contain typical Se concentrations comparative to Pyrite abundance appears variable across the sample set. High Se world Se coal content, with anomalously high content in westerly in Fife‐Stirling, Ayrshire, Cumberland, Northumberland, North Wales, coal exposures (Argyllshire, Ayrshire, Fife‐Stirling, Cumberland, and Staffordshire coincide with high S and visible pyrite in sampled Nottinghamshire, North Wales, and Staffordshire), Northumber- coals. Pyrite occurs in coal as early (syngenetic) replacement minerals, land and Fife‐Stirling. The high Se‐Bowland Shale and/or Ordovi- and later (epigenetic) cleat‐filling pyrite (Cavender & Spears, 1995, cian volcanics may have provided a source of Se (and other trace 1997; Spears & Caswell, 1986; Spears & Tewalt, 2009). Samples from elements such as Te). the South Wales, Ayrshire and Northumberland coalfields show the ‐ highest pyrite abundance, with disseminated pyrite, banded pyrite 2. Based on a combination of CRC and standard mode ICP MS and later stage cleat pyrite evident. In previous studies, Se has been methods, the mean measured Te content of British coals is ‐ shown to be more abundant in early forms of pyrite (Large et al., 0.02 mg/kg. The highest recognized Te content is from Fife Stir- 2014). However, late stage epigenetic fluids are also known to be ling, a magnitude higher than the estimated world mean Te value. responsible for high concentrations of trace elements in some coals 3. High Se in Fife‐Stirling, Ayrshire, Cumberland, Northumberland, (Bullock et al., 2018; Diehl et al., 2004; Spears & Tewalt, 2009; North Wales, and Staffordshire coincide with high S and visible Yudovich & Ketris, 2006). The presence of pyrite infilling cleats indi- pyrite in sampled coals. There is a positive correlation in the Te/ cates a later period of formation. Cleats can provide channels for fluid Se ratio, suggesting substitution for S in pyrite and an important flow, and the host coal can offer a source of reduced S for bacterial role of pyrite in sequestration of Se and Te in coals. Se and Te sulphate reduction (Bullock et al., 2018; Hatch, Gluskoter, & Lindahl, may be typically more abundant in early forms of pyrite, but the 1976). The development of cleats provides a locus for pyrite precipita- development of cleats provides a locus for pyrite precipitation tion, and associated Se and Te. and associated higher Se and Te. 1410 BULLOCK ET AL.

4. Regional variations in coal maturity and mineralization trends Desulphurisation, Institute of Chemical Engineers Symposium, Series – relating to igneous intrusions or episodes of deformation may play 138, 197 205. a role in the variations in Se and Te enrichment in British coals Cavender, P. F., & Spears, D. A. (1997). Sulphur distribution in a multi‐bed seam. In R. A. Gayer, & J. Peŝek (Eds.), European coal geology and tech- Enrichment in some areas may be additionally influenced by sea- nology, Geological Society Special Publication (Vol. 125) (pp. 245–260). water distribution and inundation into low‐lying deltaic planes. London, UK: Geological Society. Coleman, L., Bragg, L. J., & Finkelman, R. B. (1993). Distribution and mode of occurrence of selenium in US coals. Environmental Geochemistry and – ACKNOWLEDGEMENTS Health, 15, 215 227. Cravotta, C. A. III (2008a). Dissolved metals and associated constituents in The authors wish to thank Kier Group, the British Coal Utilisation abandoned coal‐mine discharges, Pennsylvania, USA. Part 1: Constitu- Research Association (BCURA) and Uniper (E.On) for kindly providing ent quantities and correlations. Applied Geochemistry, 23, 166–202. coal samples. The authors are grateful to Conselho Nacional de Cravotta, C. A. III (2008b). Dissolved metals and associated constituents in Desenvolvimento Científico e Tecnológico (CNPq), Coordenação de abandoned coal‐mine discharges, Pennsylvania, USA. Part 2: Geochem- Aperfeiçoamento de Pessoal de Nível Superior (CAPES), and Fundação ical controls on constituent concentrations. Applied Geochemistry, 23, 203–226. de Amparo à Pesquisa do Estado do Rio Grande do Sul (FAPERGS) for Dai, S., & Finkelman, R. B. (2018). Coal as a promising source of critical supporting this study. The authors are grateful for the thorough and elements: Progress and future prospects. 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